This invention relates to optical substrates and, more specifically, to thin film coated optical substrates for discriminating between the polarization states of light transmitted therethrough.
In backlight computer displays or other display systems, optical films are commonly used to direct light. For example, in backlight displays, brightness enhancement films use prismatic structures to direct light along a viewing axis (i.e., an axis substantially normal to the display). This enhances the brightness of the light viewed by a user of the display and allows the system to consume less power in creating a desired level of on-axis illumination. Films for turning light can also be used in a wide range of other optical designs, such as for projection displays, traffic signals, and illuminated signs.
Backlight displays and other systems use layers of films stacked and arranged so that the prismatic surfaces thereof are perpendicular to one another and are sandwiched between other optical films known as diffusers. Diffusers have highly irregular surfaces.
Light turning and diffusion are typically handled with a 3 or 4 film stack. The stack is comprised of brightness enhancing films and diffuser films. Polarization recycling is typically accomplished by using other films in addition to the typical stack (sometimes one of the films is replaced by this additional element). This additional film may be a multilayer birefringent film, a liquid crystal birefringent film, a birefringent film with scattering particles or a MacNielle type array of polarizing beam splitter prisms formed in a film.
A first embodiment of the invention features a polarization sensitive optical substrate which comprises a planar surface and a first thin film applied to the planar surface. The first thin film has a thickness of xcex/4/, where xcex is the wavelength in air of light incident upon the first thin film and n is the refractive index of the first thin film. A first prismatic surface, having a prescribed peak angle, xcex1, height, h, length, l, and pitch, p, is optionally also coated with a second thin film, and is in opposition to the planar surface. Yet further, the planar surface may be replaced with a second prismatic surface similar to the first prismatic surface. One or both of the prismatic surfaces may be randomized in their peak angle, xcex1, height, h, length, l, and pitch, p.
The second prismatic surface may also have a random or non-random peak angle, xcex3, height, g, length, l, and pitch, q. The prismatic surface may also comprise a refractive index different than that of the substrate.
A second embodiment of the invention features a backlight display device comprising an optical source for generating light. A light guide guides the light therealong. A reflective device, positioned along the light guide, reflects the light out of the light guide. The backlight display device includes a polarization sensitive optical substrate comprising a planar surface receptive of light from the light guide and a first thin film applied to the planar surface. The first thin film has a thickness of xcex/4/, where xcex is the wavelength of light incident upon the first thin film and n is the refractive index of the first thin film. A first prismatic surface is in opposition to the planar surface and a spacer is positioned between the polarization sensitive optical substrate and the light guide for preventing contact therebetween. The first prismatic surface, having a prescribed peak angle, xcex1, height, h, length, l, and pitch, p, is optionally also coated with a second thin film, and is in opposition to the planar surface. Yet further, the planar surface may be replaced with a second prismatic surface similar to the first prismatic surface. One or both of the prismatic surfaces may be randomized in their peak angle, xcex1, height, h, length, l, and pitch, p. The second prismatic surface may have peak angle, xcex3, height, g, length, l, and pitch, q.
The invention works by allowing highly oblique light, such as that exiting the backlight display device to enter the polarization sensitive optical substrate at a glancing angle (e.g., between 60 and 90 degrees as measured from the normal to the average surface or a nominal plane) without an intervening diffuser. The polarization sensitive optical substrate directs the incident light such that the light exiting therefrom is in a direction that is close to the average surface normal of the polarization sensitive optical substrate. This results in partial polarization of the exiting light. The polarization effect is enhanced by the use of thin film coatings applied to the surfaces of the polarization sensitive optical substrate. For example, a single thin film of xc2xc wavelength of light in thickness of a high index of refraction material such as a metal oxide such as TiO2 may be applied to a planar surface of the polarization sensitive optical substrate. An additional substrate may be located above the polarization sensitive optical substrate to provide diffusion of light. This substrate may be a retarder film that is used to rotate the plane of polarization of the light exiting the polarization sensitive optical substrate such that the light is better matched to the input polarization axis of an LCD. Alternatively, for this purpose the retarder film could be built into the lower LCD substrate.